It is sometimes useful to update the running code and / or configuration of a network service, without disrupting existing connections. Usually, this is achieved by starting a new process, somehow transferring clients to it and then exiting the old process.
There are many ways to implement graceful upgrades. They vary wildly in the trade-offs they make, and how much control they afford the user. This library has the following goals:
- No old code keeps running after a successful upgrade
- The new process has a grace period for performing initialisation
- Crashing during initialisation is OK
- Only a single upgrade is ever run in parallel
tableflip works on Linux and macOS.
upg, _ := tableflip.New(tableflip.Options{})
defer upg.Stop()
go func() {
sig := make(chan os.Signal, 1)
signal.Notify(sig, syscall.SIGHUP)
for range sig {
upg.Upgrade()
}
}()
// Listen must be called before Ready
ln, _ := upg.Listen("tcp", "localhost:8080")
defer ln.Close()
go http.Serve(ln, nil)
if err := upg.Ready(); err != nil {
panic(err)
}
<-upg.Exit()Please see the more elaborate graceful shutdown with net/http example.
[Unit]
Description=Service using tableflip
[Service]
ExecStart=/path/to/binary -some-flag /path/to/pid-file
ExecReload=/bin/kill -HUP $MAINPID
PIDFile=/path/to/pid-file
See the documentation as well.
The logs of a process using tableflip may go missing due to a bug in journald. You can work around this by logging directly to journald, for example by using go-systemd/journal and looking for the $JOURNAL_STREAM environment variable.